Several families of transcription factors, such as DREB/CBF, ERF, MYK, MYB, AREB/ABF, NAC and HDZip class I and II, have been shown to be involved in the regulation of drought response in plants. These factors bind specific cis-elements on the promoters of drought regulated genes.
The dehydration-responsive element-binding proteins (DREBs) or C-repeat-binding proteins (CBFs) are among the first discovered families of transcription factors responsible for gene regulation under conditions of water deficiency. It is a group of transcriptional factors with a single AP2 domain, which controls the expression of many stress inducible genes in plants. Many DREB/CBF factors are themselves induced by such abiotic stresses, like drought, cold, salinity, and heat.
Six DREB transcription factors, including four DREB1/CBF and two DREB2 factors have been identified in Arabidopsis. It has been demonstrated that DREB1/CBF factors are induced by drought, salt and cold, whereas DREB2 factors are induced by drought and salt only.
Since discovery of the role of DREB/CBF factors in the stress response, several attempts have been undertaken to demonstrate their potential to improve stress tolerance in Arabidopsis, and crop plants such as Brassica junceae, soybean, rice, wheat and other grasses.
In the majority of attempts to overexpress DREB factors in plants, constitutive promoters such as the Cauliflower mosaic virus 35S promoter, rice actin 1 promoter and maize polyubiquitin promoter have been used. However, in most cases strong constitutive expression led to different degrees of growth retardation, which subsequently led to dwarfism of the transgenic plants.
There have been attempts to overcome the problems of severe growth retardation by reducing the duration of overexpression using stress inducible promoters. For example expression of DREB factors under the control of the rd29A promoter has been attempted in a range of plants. However, this promoter was generally observed to have some level of basal expression in the absence of drought stress. As mentioned above, expression of DREB factors in the absence of drought stress is associated with the dwarfism and or stunting of plants.
In light of the above, it would be desirable to be able to drive the expression of nucleotide sequences, including those involved in drought tolerance (such as DREB factors), in a drought responsive manner. In this way, the nucleotide sequences could be expressed when required, ie. in response to drought, but not be expressed in the absence of drought where undesirable phenotypes such as dwarfism and or stunting of plants may occur.
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any country.